A filament winding method is to equally align the fiber bundle, wet the fiber bundle in a resin bath, regularly wind it in an assigned pattern, thickness and weight on a rotating mandrel, and cure and then strip the wound fiber bundle off the mandrel or strip off and then cure the wound fiber bundle on the mandrel.
Early products, e.g., a rocket engine cowling, a pressure vessel, a buoy, a conical tip of a missile, a storage tank, a cylindrical tube, a motor vane spring, a helicopter propeller, a spaceship spar, an airplane fuselage and other aerospace or industrial component parts, were made mainly in a cylindrical outline. After the fiber wetted with resin is wound on the mandrel in an assigned pattern, one has to cure the resin carried on the fiber with heat and then draw off the no longer required mandrel. So, the mandrel should be made of a material that is soluble or has a low melting point (but still higher than the curing temperature of the resin) in order to pre-form the outline and size of a product, e.g., a golf club. The filament winding process for making a carbon fiber golf club is to wind a fiber that has been wetted in a resin bath in an assigned pattern on a conical iron core (i.e., a mandrel) mounted in a winding machine, and then wrap a heat shrinkable tape layer on the wound fiber such that the fiber wetted with the resin can be tightly formed on the mandrel and the fiber can be cured in an oven. After stripping off the tape layer and the mandrel, the wound fiber is ground to a required outer diameter in a centerless grinding machine to obtain a prototype of the golf club. However, we cannot manufacture sporting products such as bicycle handle bars, bicycle front forks, tennis rackets or badminton rackets that have complex outlines, varying cross-sectional sizes or unsymmetrical straight cylindrical tubes by using the shrinkable tape and applying the same curing procedure as used for a golf club. Instead, one has to use a mandrel that has an inflatable bladder inside by blowing up the mandrel to pressurize, harden and form a product. Consequently, it is necessary to remove the mandrel before the wound fiber can be cured.
A conventional filament winding method is to wind a fiber wetted with resin in an assigned pattern on an extractable mandrel, cure the wound fiber and then extract the mandrel. This, however, is not suitable for the aforementioned product having a complex outline. Generally, for a product having a special requirement, the fiber is wound in an assigned pattern on a mandrel which is then heated until the resin carried on the fiber does not contaminate the operator's hand (i.e., a B-stage of resin). The wound fiber is then stripped off the mandrel and put in an oven to be cured and formed. The design of a mandrel for products having complex outlines, varying cross-sectional sizes and unsymmetrical straight cylindrical tubes is therefore important. Conventional extractable mandrel designs, e.g., illustrated in U.S. Pat. Nos. 4,448,624, 4,684,423 and 5,259,901, are different from the present invention in both the configuration and the usage. Moreover, the mandrel used in a prior art manufacturing process, as shown in FIG. 1, has a uniformly cross-sectional iron core 10 encapsulated with two or three long arc strip shells 11. After sleeving an inflatable bladder on such a mandrel, one may wind a fiber wetted with resin in an assigned pattern on the mandrel that is mounted in a winding machine. Thereafter, the wound fiber, the arc strip shells 11 and the iron core 10 are put into an oven to have the resin carried by the fiber cured to a B-stage. The iron core 10 is then removed by a rapping device to leave a long hollow cylindrical configuration having inner arc shells 11, an inflatable bladder and an outer wound fiber. The arc shells 11 is then extracted to leave a final cylindrical strip with the wound fiber encapsulated around the inflatable bladder. After putting the final cylindrical strip into an open mold and closing the mold, one may blow air into the final cylindrical strip to pressurize, heat and cure the product. Even though the iron core 10, the arc shells 11 and the inflatable bladder can be designed specifically according to the size or outer diameter of a product, the design of the mandrel is very important due to the fact that it is very difficult to strip off a mandrel which is tightly wound by fiber with a high tension. The prior art manufacturing process which uses an extractable mandrel can only strip off a mandrel by a rapping device and has the following defects:
1. It can only form a uniformly cross-sectional configuration;
2. It requires the use of a large number of iron cores and arc shells for mass production; and
3. It requires a rapping device which is costly.